The present invention relates to the diagnosis and treatment of partial or complete upper airway occlusion, a condition in which the upper airway collapses, particularly under the reduced pressure generated by inhalation. This is most likely to happen during unconsciousness, sleep or anesthesia.
A particular application of the present invention is to the diagnosis and/or treatment of snoring and sleep apnea. Sleep apnea is characterized by complete occlusion of the upper airway passage during sleep while snoring is characterized by partial occlusion. An obstructive sleep apnea sufferer repeatedly chokes on their tongue and soft palate throughout an entire sleep period, resulting in lowered arterial blood oxygen levels and poor quality of sleep. It should be realized that although the following specification discusses sleep apnea in detail, the present invention also applies to the diagnosis and treatment of other forms of upper airway disorders.
The application of continuous positive airway pressure (CPAP) has been used as a means of treating the occurrence of obstructive sleep apnea. The patient is connected to a positive pressure air supply by means of a mouth and nose mask, nose mask only or nasal prongs. The air supply breathed by the patient is at all times at slightly greater than atmospheric pressure. For example, therapeutic pressures will typically be within the range of 4 cmH2O to 20 cmH2O. It has been found that the application of continuous positive airway pressure (CPAP) provides what can be described as a “pneumatic splint”, supporting and stabilizing the upper airway and thus eliminating the occurrence of upper airway occlusions. It is effective in eliminating both snoring and obstructive sleep apnea, and in many cases is effective in treating central and mixed apnea.
The airway pressure required for effective CPAP therapy differs from patient to patient. In order to discover the airway pressure which is most effective for a particular individual, the practice has been for the patient to undergo two sleep studies at an appropriate observation facility such as a hospital, clinic or laboratory. The first night is spent observing the patient in sleep and recording selected parameters such as oxygen saturation, chest wall and abdominal movement, air flow, expired CO2, ECG, EEG, EMG and eye movement. This information can be interpreted to diagnose the nature of the sleep disorder and confirm the presence or absence of apnea and, where present, the frequency and duration of apneic episodes and extent and duration of associated oxygen desaturation. Apneas can be identified as obstructive, central or mixed. The second night is spent with the patient undergoing nasal CPAP therapy. When apnea is observed, the CPAP setting is increased to prevent apneas. The pressure setting at the end of the sleep period, i.e., the maximum used, is deemed to be the appropriate setting for that patient.
For a given patient in a given physical condition, various stages of sleep will require different minimum pressures to prevent occlusions. Furthermore, these various pressures will, in fact, vary from day to day depending upon the patient's physical condition, for example, nasal congestion, general tiredness, and effects of drugs such as alcohol, as well as the patient's sleeping posture. Thus the appropriate pressure found in the laboratory is necessarily the maximum of all these minimum pressures for that particular night and is not necessarily the ideal pressure for all occasions nor for every night. It will generally be higher than necessary for most of the night.
Also, a patient must be able to operate a CPAP system to deliver appropriate airway pressure at home where their general physical condition or state of health may be quite different from that in the sleep clinic, and will certainly vary from day to day. The patient's physical condition often improves due to CPAP therapy. It is often the case that after a period of therapy the necessary airway pressure can be reduced by some amount while still preventing the occurrence of obstructive sleep apnea.
The long term effects of CPAP therapy are unknown so it is desirable to keep the airway pressure as low as practicable, particularly if a patient requires long term treatment. Lower airway pressures also result in a lower face mask pressure which is generally more comfortable for the patient. It has been found that CPAP induces patients to swallow and this inducement to swallow can be reduced by lowering the airway pressure. Thus it is desirable to use the lowest practicable airway pressure that is effective in preventing airway occlusion during CPAP therapy for the comfort and possibly the long term safety of the patient. Also, a lower airway pressure requires less energy consumption and a less complex and therefore less expensive apparatus, which is also generally quieter.
Low airway pressures are also desirable before and during the early stage of each sleep period as the increased comfort of an initially lower airway pressure allows the patient to more easily fall asleep. When a patient undergoing CPAP opens their mouth with pressurized air being forced through the nose, the pressured air exits out of the mouth producing an unpleasant sensation. This can occur when the patient puts on the mask connected to the pressured air supply before falling asleep and some patients will therefore leave the mask off for as long as possible and may in fact fall asleep without wearing the mask and therefore without the benefits of the CPAP therapy.
In addition to the problems associated with administering CPAP therapy there exists the inconvenience and cost of diagnosis which may be undertaken by overnight observation at a sleep clinic or the like. Hence a simple means whereby a patient's apnea problem can be diagnosed at home without supervision is clearly desirable as well as a CPAP device which will deliver a continuously minimum appropriate pressure for substantially the entire period of therapy.
Although diagnosis in a sleep clinic as outlined above is beneficial, it has some deficiencies. A patient is likely not to sleep in a fully relaxed state in an unfamiliar environment and a single night is insufficient to obtain a pressure setting that will be optimal in the long run. Thus home therapy at the pressure setting arrived at in this way is likely to be less than 100% effective on some occasions and higher than necessary for a substantial portion of the time. The cost and inconvenience of a sleep study in a hospital setting are to be avoided if possible.
A skilled physician can usually recognize the symptoms of sleep apnea from questioning and examining a patient. Where no other indications are present there is very little risk in attempting nasal CPAP therapy without further testing as the treatment is fail safe and non-invasive. However, a very useful intermediate step would be to analyze the pattern of respiratory waveforms (e.g., pressure, flow or sound) over one or more full nights of sleep. Interpretation of these patterns together with questioning and examination will, in many cases, provide sufficient confirmation of apnea to prescribe nasal CPAP therapy. If nasal CPAP eliminates the symptoms of day time sleepiness (as assessed by the patient) and of apneic snoring patterns (as assessed by analysis of recorded respiratory sounds while on nasal CPAP), the treatment can be continued. Further check-ups can be conducted at intervals recommended by the physician.
In the most general form of a CPAP treatment device, the intermediate step before the device attempts CPAP pressure increases is to analyze the patterns of the respiratory parameters that can be obtained from sensors, such as a pressure sensor or flow sensor. As those skilled in the art will recognize, these parameters include, in addition to acoustic rate of breathing, inhaled/exhaled air volume and inhaled/exhaled air flow rate, and provide comprehensive information for the physician to assess the patient's condition. This additional information, for example, generated by a pressure transducer, is available at additional cost and complexity. Similar information related to airflow may be estimated from the speed of or current supplied to the blower of the apparatus that is supplying the pressure to the mask in a system where pressure changes are generated by changing the speed of the blower. Examples of such an implementation are disclosed in commonly owned U.S. Pat. Nos. 5,740,795, 6,332,463 and 6,237,593, the disclosures of which are hereby incorporated by reference.
The measurement of other parameters would provide further information to assist diagnoses, and the acoustic and/or other respiratory recordings described above can readily be used in conjunction with other monitors such as ECG and/or pulse oximetry. Suitable monitors are available to measure both these parameters in the home. The correlation between reduced oxygen saturation and apnea is sufficiently well established to infer oxygen desaturation from the confirmation of an apneic event.
One index determined from these parameters is the Apnea Hypopnea Index. The Apnea Hypopnea Index (“AHI”) is an indicator of severity of a patient's sleep disordered breathing. The AHI is determined by adding the total number of apneas and hypopneas the patient experienced over a particular time period, such as during a sleep clinic study. Various forms of AHI index are known by those skilled in the art.
However, in automated devices, sophisticated sensors and associated algorithms for detecting SDB events and determining an appropriate response to the detected events add a level of complexity to the device that may increase the cost, potentially making them too expensive for some patients. Thus there is a need for a device that can accurately adjust the therapeutic pressures in response to SDB events to alleviate the events but utilizing minimum hardware and minimized methodology for controlling the hardware.